This game is also available on N64.
Go back to the GameCube game. Once back in your town, leave the screen with the boat and come back. Talk to the Kappa again and the GCN game will be updated with the new data from the GBA island. Travel back to the island and collect all the money bags. Some bags contain as much as 30,000 coins! You should be able to pay off your house in no time now.
Note that planting a money bag just anywhere will not produce a money tree. It has to be planted in the sparkly special spot.
Red furniture in the East
Green furniture in the South
What does this do? For example, the yellow furniture increases the amount of money you receive. The other colors have similar benefits, so be sure to follow this basic rule when constructing your house.
Enter the monthly lottery at the Tanuki Store on the last day of the month. You get a lottery ticket for every major purchase you make at the store (stationary and plants don't count). Every five tickets let you play in the lottery once, so be sure to collect a bunch of them. Note that lottery tickets don't carry over to the next month. They're only good for the current month. Prizes in the lottery vary from month to month, so don't give up if you don't find any NES games the first time you enter the lottery.
Check your mailbox on Christmas Eve. Every player gets an NES game as a Christmas present.
Get a game for your birthday. Don't only ask animals if you can run chores for them -- use the second "conversation" option as well. At one point, one of your animals will ask you for your birthday and blood type. This animal will drop by your house on your birthday and personally deliver a new Famicom game.
There are other ways to get your hands on emulated classics, so keep your eyes and ears open.